The present invention relates to a negative resistance element utilizing a resonant tunneling phenomenon in a semiconductor multi-barrier structure, and more particularly to a resonant tunneling device which can provide a negative resistance characteristic having substantially uniform or equal peak current values and can be used as a multiple (three or more) state logic element.
The conventional tunneling device employs a double-layer structure in which a semiconductor layer (or thin film) having a narrow energy bandgap is sandwiched by two semiconductor layers each having a wide energy bandgap, as is disclosed by the article entitled "Room Temperature Observation of Differential Negative Resistance in an AlAs/GaAs/AlAs Resonant Tunneling Diode", Japanese Journal of Applied Physics, Vol. 24, pp. L466-L468, 1985 and the article entitled "Observation of Resonant Tunneling Via First Excited Level in Double-Barrier Diode", Electronics Letters Vol. 22, No. 8, pp. 406-407, 1986, or a triple-barrier structure which includes an interlamination of two semiconductor layers each having a narrow energy bandgap three semiconductor layers each having a wide energy bandgap, as is disclosed by the article entitled "Observation of resonant tunneling in AlGaAs/GaAs triple barrier diodes", Applied Physics Letters, Vol. 49, pp. 73-75, 1986.
FIGS. 2A and 2B show energy band diagrams for the conduction band for the above-mentioned double-barrier and triple-barrier structures respectively. In the double-barrier structure shown in FIG. 2A, quantum levels E.sub.0, E.sub.1,--are formed in a quantum well layer 5 between first and second barrier layers 1 and 2 and a peak appears in a tunnel current each time the quantum level becomes equal to a quasi-Fermi level E.sub.F l on the cathode side through application of a voltage V. In the triple-barrier structure shown in FIG. 2B, current peaks are produced by the tunneling from a quantum level E.sub.0.sup.1 formed in a first quantum well layer 5 between first and second barrier layers 1 and 2 (or a reference level for injection of electrons) to quantum levels E.sub.0.sup.2, E.sub.1 2,--formed in a second quantum well layer 6 between second and third barrier layers 2 and 3, that is, E.sub.0.sup.1 .fwdarw.E.sub.0.sup.2, E.sub.0.sup.1 .fwdarw.E.sub.1.sup.2,--.
However, the current versus voltage characteristic of each of the conventional tunneling devices exhibits remarkably different peak current values, as shown in FIG. 3, and hence it is difficult to use it as a multiple state logic element. Also, each of peak voltages V.sub.1, V.sub.2,--is determined by quantum levels formed in one quantum well and hence it is difficult to control the peak voltages independently from each other. Accordingly, a solution of these problems is desired.